This invention relates generally to a voltage regulator and more particularly, to a shunt voltage regulator utilizing a reference voltage generator which generates a floating output voltage with respect to the voltage of the power supply.
Referring to FIG. 1, there is shown a prior art shunt voltage regulator 10. In FIG. 1, a power supply V.sub.1 generates a voltage such as 15 volts which due to temperature or load variations might have some fluctuations. In order to create a constant voltage, the shunt voltage regulator 10 is needed. In this example, in addition to regulating the voltage (creating a constant voltage), the output voltage V.sub.OUT1 is also lowered to 5 volts in order to supply a constant 5 volts to a CMOS circuitry.
The shunt voltage regulator 10 comprises a reference voltage generator 14, an Op-Amp 16, a Metal Oxide Silicon Field Effect Transistor (MOSFET) T.sub.1 and two resistors R.sub.2 and R.sub.3. The negative terminal of the reference voltage generator 14 is grounded and the positive terminal of the reference voltage generator 14 is connected to the inverting input (-) of the Op-Amp 16. The output of the Op-Amp 16 is connected to the gate of the of transistor T.sub.1. The source of transistor T.sub.1 is grounded and the drain of transistor T.sub.1 is connected to an output node 12. The non-inverting (+) input of the Op-Amp 16 is connected to node 12 through resistor R.sub.2 and also grounded through resistor R.sub.3. In addition, the power supply V.sub.1 is connected to the output node 12 through resistor R.sub.1.
In FIG. 1, the output voltage V.sub.OUT1 at node 12 is equal to: EQU V.sub.OUT1 =(R.sub.2 +R.sub.3)I.sub.1 =(R.sub.2 +R.sub.3).V.sub.3 /R.sub.3 !=(1+R.sub.2 /R.sub.3)V.sub.3.
Also, since Op-Amp 16 is used in linear mode, the voltage of the non-inventing input is set to be equal to the voltage of the inverting input which is equal to the output voltage of the reference voltage generator 14. The reference voltage generator 14 generates a reference voltage V.sub.R of 1 volt. Therefore, both voltages of the inverting and the non inverting inputs of the Op-Amp 16 are equal to 1 volt. Therefore, since EQU V.sub.3 =V.sub.R,
then EQU V.sub.OUT1 =(1+R.sub.2 /R.sub.3)V.sub.3 =(1+R.sub.2 /R.sub.3)V.sub.R.
The above relationship indicates that the shunt voltage regulator 10 keeps the output voltage V.sub.OUT1 independent of input voltage V.sub.1 and proportional to the reference voltage V.sub.R from the reference voltage generator 14. The shunt voltage regulator 10 regulates the output voltage V.sub.OUT1 and compensates for any variation in the voltage of the power supply.
For example, if the power supply V.sub.1 fluctuates from 15 volts to 16 volts, the output voltage V.sub.OUT1 tends to increase. Once the output voltage V.sub.OUT1 momentarily changes, the voltage of the non-inverting input of the Op-Amp 16 increases. The difference between the two inputs of the Op-Amp 16 increases the gate voltage of the transistor T.sub.1 which in turn increases the current drawn from T.sub.1 and R.sub.1. The increase in the current of T.sub.1 and resistor R.sub.1 will decrease the voltage of node 12. This continues until the voltage V.sub.1 and hence the output voltage V.sub.OUT1 return back to original values.
By selecting proper R.sub.2 and R.sub.3, a desired output voltage V.sub.OUT1 can be selected. For example in FIG. 1, R.sub.2 and R.sub.3 are selected to set the output voltage at node 12 to 5 volts. In this circuit, since the reference voltage V.sub.R of the reference voltage generator 14 is temperature insensitive, the output voltage V.sub.OUT1 is also temperature insensitive.
Typically, shunt voltage regulators utilize reference voltage generators to create a fixed voltage at the inverting and the non-inverting inputs of the Op-Amp to generate a fixed voltage at the output node. However, due to the popularity of the CMOS process and in particular P-substrate CMOS process, it is desirable to design a reference voltage generator using bipolar transistors fabricated with P-substrate CMOS technology. Fabricating a bipolar transistor in P-substrate CMOS technology is well known in the industry. Yet, designing a reference voltage generator with bipolar transistors in P-substrate CMOS technology creates a temperature dependent reference voltage with respect to the power supply.
The transient variation of the voltage of the power supply causes the output of the reference voltage generator to vary (float). A typical voltage generator is designed to generate a reference voltage with respect to the ground of the integrated circuit and therefore, the voltage is substantially fixed as the power supply voltage or the temperature varies.
The reason a reference voltage generated by P-substrate CMOS technology has a floating voltage is that the bipolar transistors fabricated by P-substrate CMOS technology are PNP transistors. In order to generate a reference voltage with respect to the ground, NPN transistors are required which can be easily fabricated in N-substrate CMOS technology.
Referring to FIG. 2, there is shown a bipolar transistor 20 fabricated with P-substrate CMOS technology. In P-substrate CMOS technology, the substrate which is a P-substrate is typically connected to ground or the most negative voltage used in the integrated circuit. Therefore, in P-substrate CMOS technology, in order to create a bipolar transistor, the bipolar transistor has to be created in a well. Since the substrate is a p-substrate, the well has to be n-well which then dictates that the bipolar transistor to be a PNP transistor. In this type of configuration, n-well is used as the base B, one of the p+ regions is used as collector C and the other p+ region is used as the emitter E of the bipolar transistor 20.
In FIG. 2, layer 22 is an insulator and layer 24 is a material such as aluminum to be used for the gate G of a P-substrate CMOS transistor. Since the transistor 20 is used as a bipolar transistor, gate G is connected to a voltage above 5 volts which does not affect the function of bipolar transistor 20.
Referring to FIG. 3, there is shown a block diagram of a reference voltage generator 30 built with NPN transistors which generates a fixed 1 volt reference voltage. The reference voltage 1 volt is generated with respect to ground and since the voltage of ground is designated as zero, the output voltage V.sub.R1 of the reference voltage generator 30 is a fixed 1 volt.
Referring to FIG. 4, there is shown a block diagram of a reference voltage generator 40 built with PNP transistors which generates 1 volt. The reference voltage generator 40 generates a fixed 1 volt reference voltage with respect to power supply V.sub.2 and since the voltage of the power supply V.sub.2 is typically 5 volts, the output V.sub.R2 of the reference voltage generator 40 is 5-1=4 Volts. The output of the reference voltage generator 40 is floating since any transient change in the power supply causes the output voltage V.sub.R2 to vary. For example, if the voltage of the power supply changes to 5.2, then the output V.sub.R2 is 5.2-1=4.2 Volts.
Therefore, in this specification the term "floating" shall mean "a voltage which is a fixed voltage below the voltage of a power supply and therefore follows the transient changes of the power supply". Furthermore, in this specification "floating reference voltage generator" shall mean a reference voltage generator which generates a floating output voltage such that the difference between the voltage of the power supply and the floating output voltage is a fixed voltage independent of temperature variations.
It is an object of this invention to provide a shunt voltage regulator which utilizes a floating reference voltage generator.